This invention relates to a magnetic recording and reproducing apparatus having mounted thereon magnetic heads (MR magnetic heads) including a magnetoresistive effect element (MR element), and more particularly to a positioning technique in the magnetic recording and reproducing apparatus whereby a magnetic head traces a desired magnetic track (a following action), then starts moving to another desired track (a seek action).
One technique to improve the positioning accuracy of the magnetic heads for the purpose of increasing the magnetic track density, is to use a data track servo (also referred to as "data surface servo") that uses positioning information recorded in advance on the data surfaces of a magnetic recording medium. When one is going to apply the MR magnetic head technique to this data track servo technique, it is necessary to improve the durability of the MR magnetic heads.
This will be described in detail.
If the recording density in the magnetic recording and reproducing apparatus is to be increased, it is unavoidable to reduce the magnetic transition region per unit area on the magnetic medium. This means to decrease the width of each magnetic track (the distance perpendicular to the advancing direction of a magnetic head over the track). Accordingly, the reproduction output of the magnetic head is decreased. A realistic measure to prevent this decrease of reproduction output is to improve the sensitivity of the magnetic head. Under the circumstances, the MR magnetic head including a magnetoresistive effect element (MR element), which is a read-only head for reading the magnetic transition region, has come to be used.
As means for improving the sensitivity of the magnetic heads, if an attempt is made to obtain a large reproduction output by supplying a large sense current to the MR element, the following technical problem arises.
Generally, the sense current density of the MR magnetic head is so high, at somewhere around 10.sup.7 A/m.sup.2, that the life of the MR element tends to be short; for example, electromigration occurs in the material or wiring of the MR element, and a consequent rise of temperature causes the electromigration to be accelerated, resulting in a breakdown of the element.
In order to increase the magnetic track density, it is necessary to improve the positioning accuracy of the magnetic heads. This is the reason why data track servo has come to be used. In the data track servo technique, head positioning information recorded on the data surfaces is used. A combined use of this data track servo technique with MR magnetic heads give rise to the following technical problem.
Generally, it is necessary to make the magnetic head performs a following action to trace a magnetic track. In the data track servo technique, it is necessary to constantly reproduce, with the magnetic head, position information recorded at the leading portions of the sectors obtained by subdividing the tracks in fan-shaped blocks. When data has been reproduced by a magnetic head, the same magnetic head continues a following action while reproducing position information until the next record or, reproduce order is given. In a magnetic recording and reproducing apparatus used with a low frequency of reproduction, it often takes some time before the next record or reproduce order is given, with the result that a cumulative time of use of the MR magnetic heads, including reproduction of position information, is prolonged. This has a direct bearing on the shortening of the life of the MR magnetic heads.
An object of the present invention is to lengthen the life of the MR magnetic heads by minimizing the time of supplying the sense current to the magnetoresistive effect elements of the MR magnetic heads when the magnetic recording and reproducing apparatus is in a state waiting for a data access command from the host computer.
Techniques for prolonging the life of the MR magnetic heads are known. This is a technique, disclosed in Japanese Patent Application No. 5-226850 (filed on Sep. 13, 1993), that controls the head positioning time such that the cumulative time of use of each MR magnetic head, which is a sum of positioning time, data reproduction time, etc. is a fixed length of time. In the present invention of this patent application, however, the above-mentioned problem is to be solved by a different technique.